Organic lens molds in inorganic glass and novel inorganic glasses

ABSTRACT

The principal object of the present invention is organic lens molds, constituted wholly or in part of at least one specific inorganic glass, advantageously strengthened by a chemical tempering or a thermal tempering. Said glass has the following composition, expressed in percentages by weight: SiO2 56-66, Al2O3 2.5-10, B2O3 0.5-7, Li2O 0-3, Na2O 8-15, K2O 3-12, with Li2O+Na2O+K2O 12-20, ZnO 2-12, MgO 0-3, TiO2 0-0.5, ZrO2 1-9, CaO 0-1, BaO 0-2, SrO 0-2, with MgO+CaO+SrO+BaO 0-5, Cl 0-0.5, As2O3+sb2O3 0-1. The invention also deals with novel inorganic glasses which have the above composition with a single exception relative to the Al2O3 content: Al2O3 2.5-4.

The present application is 371 of PCT/EP98/02806, filed May 6, 1998,which claims the benefit of U.S. Provisional Patent Application SerialNo. 60/051,599, filed Jul. 2, 1997, and of French Patent Application No.97 05631, filed May 7, 1997.

The principal object of the present invention is novel organic lensmolds constituted wholly or in part of at least one specific inorganicglass. Said specific inorganic glass, when novel per se, constitutes afurther object of the present invention.

The organic lenses are conventionally obtained by radical polymerizationof a polymerizable composition cast between two complementary parts of aglass mold. At least one of these two parts has an internal surfacewhich has an optical quality that confers an adequate surface quality tothe cast lens.

Heretofore, the glasses used for constituting said organic lens moldshave not in general been developed particularly to this end. Insofar astraditionally, the manufacturers of organic lenses are or have beenglass lens manufacturers, the materials used for said molds for saidorganic lenses are in commercially available ophthalmic glasses. Suchglasses have generally been treated thermally or chemically in order tostrengthen their mechanical properties.

The inventors have summarized prior art inorganic glass compositions incolumns 1 to 10 of the Table below. In double column 11 of said Table,the (novel and not novel) compositions of the glasses for the lens moldsaccording to the invention are found, the composition that shall bereverted to in more detail a little further on in the text. Theoriginality of the invention is demonstrated in considering said Table 1and the comments relating thereto below.

TABLE 1 REFERENCE N° 1 2 3 4 5 11 INVENTION Composition US-A- US-A-US-A- US-A- US-A- Glass for Glass (% by weight) 3,790,260 4,042,4054,036,623 4,015,045 4,012,131 molds per se SiO₂ * * 60-75 50-58 * 56-6656-66 P₂O₅ 5-15 Al₂O₃ 1-5 0-5 0-7 7-17 (0.2-0.5) 2.5-10 2.5-4 B₂O₃ 0-20.5-7 0.5-7 Al₂O₃ + B₂O₃ Li₂O 0 0-5 0-5 0 0-3 0-3 Na₂O * 4-15 5-10 8-233-11 8-15 8-15 K₂O * 3-15 5-10 0-15 7-15 3-12 3-12 Total M₂O 12-20 12-2013-25 12-20 12-20 12-20 ZnO * 0-15 2-8 * 2-12 2-12 MgO * 0-4 0-2 0-6.5 *0-3 0-3 ZnO + MgO 3-11 TiO₂ * 0-5 0-2 0-7 0-0.5 0-0.5 ZnO + MgO + TiO₂10-20 ZrO₂ 0-5 0-2 1-4.5 (1-5) 1-9 1-9 MgO + TiO₂ + ZrO₂ CaO 0 0-5 0-10(3-6) 0-1 0-1 MgO + Al₂O₃ 0.2-5 La₂O₃ 0-15 ZnO + La₂O₃ + CaO + MgO 8-15MgO + CaO 2.5-10 TiO₂ = MgO + CaO 4-14 BaO 0-2 0-2 SrO 0-2 0-2 CaO +ZnO + MgO + BaO SrO + BaO MgO + CaO + SrO + BaO 0-5 0-5 MgO + CaO +SrO + BaO + ZnO + ZrO₂ NaCl 0-0.5 0-0.5 Sb₂O₃ 0-2 CeO₂ 0-4.5 As₂O₃ 0-1.5As₂O₃ + Sb₂O₃0-1 As₂O₃ + Sb₂O₃0-1 CTE (10⁻⁷/° C.) 80-95 StrainPoint >495° C. Softening Point <810° C. Compression layer depth >100μm >120 μm >60 μm >70 μm REFERENCE N° 6 7 8 9 10 11 INVENTIONComposition US-A- EP-A- DE-A- GB-A- US-A- Glass for Glass (% by weight)4,259,118 0,600,302 4,325,656 2,299,991 3,951,671 molds per se SiO₂61.6-79.5 55-65 73-78 50-65 58-67 56-66 56-66 P₂O₅ Al₂O₃ 2.5-14 4-101.5-4 5-15 4.5-8 2.5-10 2.5-4 B₂O₃ 1-10.5 5-20 9-12 0-6 0.5-7 0.5-7Al₂O₃ + B₂O₃ 14-26 Li₂O 0-3 0-4 0 0-3 0-3 Na₂O 1.5-6 6-18 1-5 2-7 8.5-178-15 8-15 K₂O 2-10 1-5 4-9 2-11 3-12 3-12 Total M₂O 13-22 5-7 7-14 12-2012-20 ZnO 0-12 0-1.5 1-2 0-10 2-12 2-12 MgO 0-3.19 0-4 0-3 * 0-3 0-3ZnO + MgO TiO₂ 0-4 0-5 0-0.5 0-0.5 ZnO + MgO + TiO₂ ZrO₂ 0-1.5 0-7 0.5-31-6 0-6.5 1-9 1-9 MgO + TiO₂ + ZrO₂ 0-10 CaO 0-4.2 1-3 * 0-1 0-1 MgO +Al₂O₃ La₂O₃ ZnO + La₂O₃ + CaO + MgO MgO + CaO TiO₂ = MgO + CaO BaO0-9.6 * * 0-2 0-2 SrO * * 0-2 0-2 CaO + ZnO + MgO + BaO 3.2-17.9 SrO +BaO MgO + CaO + SrO + BaO 12-25 0-5 0-5 MgO + CaO + SrO + BaO + ZnO +ZrO₂ 6-10 NaCl 0-0.75 0-0.5 0-0.5 Sb₂O₃ 0-1 CeO₂ As₂O₃ 0-0.5 0-0.5 0-1As₂O₃ + Sb₂O₃0-1 As₂O₃ + Sb₂O₃0-1 CTE (10⁻⁷/° C.) 33.9-53.7 70.8-96.730-60 80-95 Strain Point Tg 566- 450-503° C. >495° C. 660° C. SofteningPoint 821-845° C. 688-706° C. >830° C. <810° C. Compression layerdepth >10 μm >70 μm

The ophthalmic glasses described in U.S. Pat. No. 3,790,260 (Ref. 1) areglasses of high strength, strong UV absorption (insofar as TiO₂ is amajor constituent of it). They are free from ZrO₂ and CaO.

The U.S. Pat. No. 4,042,405 (Ref. 2) describes a phosphosilicate glasshaving a compression layer depth greater than 120 μm.

The U.S. Pat. No. 4,036,623 (Ref. 3) describes white glasses known as“crown” glasses which are strengthened by a specific treatment whichincludes:

a pre-heating of said glasses,

then, an immersion of these glasses into a molten KNO₃ bath at atemperature higher than the strain point of said glasses (advantageouslyat a temperature between 510 and 710° C.); this without inducing opticaldistortion.

Said white glasses do not contain any B₂O₃ and generally comprise CaO.

The U.S. Pat. No. 4,015,045 (Ref. 4) describes glass compositions whichare perfectly appropriate for producing flat glasses. Said compositionscontain a significant amount of TiO₂ and do not contain any ZnO.

The U.S. Pat. No. 4,012,131 (Ref. 5) describes a glass for ophthalmiclenses, strengthened by an ion exchange technique, which has acompression layer depth greater than 60 μm. Said glass does not containany ZnO.

The U.S. Pat. No. 4,259,118 (Ref. 6) describes thermally pre-stressableglasses which have a low linear coefficient of thermal expansion andhigh strain point and softening point. Said glasses do not contain anyK₂O.

The EP application EP-A-0 600 302 (Ref. 7) describes fast strengthenableboroaluminosilicate glass lenses (strengthenable in less than 4 hours).Said glass contains a large amount of B₂O₃, a small amount of ZnO(advantageously, it does not contain any ZnO) and can be ZrO₂-free.

The German application DE-A-4 325 656 (Ref. 8) describes glass fireprotection articles in a glass which is very rich in silica. No mentionis made in this document of the applications of said glass in theoptical and/or ophthalmic molds or lens fields.

Finally, the GB application GB-A-2 299 991 (Ref. 9) describes achemically strengthened aluminosilicate glass for magnetic disks of lowthickness. Said glass does not contain any B₂O₃, it contains Na₂O at amaximal amount of 7% and the minimal amount of interveningMgO+CaO+BaO+SrO is more than 5%.

The use of these prior art inorganic glasses, which are different fromthose of the invention, has never been described nor suggested as moldsfor organic lenses.

The U.S. Pat. No. 3,951,671 (Ref. 10) describes glass compositions foruse in making ophthalmic lenses or lens blanks which can subsequently betoughened by an ion-exchange process. The use of said glass compositionsas molds for organic lenses is neither described nor suggested.

Within the particular context of molds for organic lenses, CORNING,SCHOTT and HOYA have especially used glasses known under the code-names:

QE-8092 (Danville), LJ-8361 (Danville), TRC 33 (Bagneaux) and BL, forCORNING (the first of these 4 glasses, referenced QE-8092, described inthe U.S. Pat. No. 3,790,260, was placed on the market in the 1970's. Itwas created in order to have a better impact resistance than the secondof said 4 glasses, referenced LJ-8361, itself placed on the market inthe 1940's. To this end, said glass QE-8092 undergoes a chemicalstrengthening);

CHW-0991 and S-3 for SHOTT;

N-4 for HOYA.

Amongst these 7 glasses, only that referenced CHW-0991 is not a standardophthalmic glass but is a glass developed specifically for preparingorganic lens molds. Its properties are nevertheless very close to thoseof such a standard ophthalmic glass: QE-8092. The compositions of saidtwo glasses—CHW-0991 and QE-8092—are in fact very close. Said twoglasses contain significant amounts of TiO₂ (about 0.8%).

The same applies for the two other glasses referenced TRC 33 and S-3.They contain 1.5 and 0.6% TiO₂ respectively.

The LJ-8361 glass itself contains 0.4% TiO₂ and 8.4% CaO.

The N-4 glass itself has a high Al₂O₃ content (14% by weight), Li₂O(4.1% by weight), CaO (2.6% by weight) and does not contain any ZnO.

The BL glass the standard “crown” white glass itself contains about 70%SiO₂, 9% CaO and only 0.7% Al₂O₃. This glass is thermostrengthenable.

Within the context of inorganic glasses in general and inorganic glassesused for organic lens molds in particular, the inventors preside novelorganic lens molds constituted wholly or in part of at least onespecific inorganic glass which is free or almost free from TiO₂ andwhich is perfectly suitable as a material for such organic lens molds.Said novel organic lens molds constitute the principal object of thepresent invention. The specific inorganic glasses likely to enter intotheir composition constitute a further object of the present inventionwhen they are novel.

The organic lens molds of the invention are characteristicallyconstituted wholly or in part of at least one inorganic glass having thefollowing composition, expressed in percentages by weight:

SiO₂ 56- 66 Al₂O₃ 2.5-10  B₂O₃ 0.5-7   Li₂O 0- 3 Na₂O   8- 15 K₂O   3-12 with Li₂O + Na₂O + K₂O 12- 20 ZnO   2- 12 MgO 0- 3 TiO₂   0- 0.5 ZrO₂1- 9 CaO 0- 1 BaO 0- 2 SrO 0- 2 with MgO + CaO + SrO + BaO 0- 5 Cl   0-0.5 As₂O₃ + Sb₂O₃  0- 1.

(The + in the expression As₂O₃+Sb₂O₃ must be read: and/or).

According to an advantageous variant, said inorganic glasses areTiO₂-free. Within the context of this advantageous variant, theparticularly preferred glass compositions for the lens molds of theinvention are given below:

SiO₂ 61.5-63   Al₂O₃ 2.5-4   B₂O₃ 0.5-3   Li₂O   0-0.5 Na₂O  8-15 K₂O 5-10 ZnO  7-12 MgO 0-3 ZrO₂ 1-7 CaO 0-1 BaO 0-2 SrO 0-2 Cl   0-0.5As₂O₃ + Sb₂O₃  0-1.

These glasses (constitutive of the lens molds according to theinvention) whose interesting properties are developed further on in thepresent text (it may already be mentioned that they are transparentwhite glasses which have a good UV transmission, a low sensitivity tosaid UVs, an interesting chemical durability and which are (easily)mechanically strengthenable) may advantageously be strengthenedmechanically by methods of chemical or thermal tempering. Such methodsare methods known per se.

A chemical tempering, advantageously carried out under specificconditions (in any case different from those described in the U.S. Pat.No. 4,036,623), is most particularly recommended for the reinforcementof these glasses.

During the method referred to as thermal tempering, the glass is, in amanner known per se, heated above its annealing point (typically, forthe glasses in question, at temperatures corresponding to10^(10.2)-10^(9.2) poises) and is then abruptly cooled by jets of air.The low thermal conductivity of the glass makes it that the core layerscongeal and retract after the shallow layers, thus placing the latterlayers under compression.

The profile of constraints obtained is parabolic with a core extensionequal to half the surface compressions. The level of constraints isproportional to the strengthenability and the thickness of the glass.

The strengthenability of the glass is itself proportional to the CTE(dilation coefficient) of the glass and to its Young's modulus and isinversely proportional to the thermal conductivity of the glass.

The glasses constitutive of the lens molds according to the inventionare particularly suited to the thermal tempering (by virtue of theirhigh CTEs and Young's moduli, as well as by virtue of their softeningpoint lower than 810° C.: see later on): they can thus be treated withinthe traditional temperature ranges.

The chemical tempering allows mechanically strengthening a glass, forexample, a glass lens, by compressing the surface of said glass. Thisresult is obtained by an ion exchange mechanism; the glass beingimmersed in a molten salt bath at a given temperature. Under the effectof the temperature, an exchange takes place between the alkali ions(Na⁺, Li⁺), which leave the surface of the glass, and those which arelarger (generally K⁺) which are present in the molten salt, which thenpenetrate the glass. After cooling, the surface of the treated glass isplaced under compression compared to the core of said glass, this thusinduces a strengthening of said glass by an increase of its resistanceto breakage. The compressed layer thus formed is uniform.

According to the prior art, in the case of the “crown” white glasses andfixed tint lenses, the chemical tempering is carried out for sixteenhours in a bath composed of 99.5% potassium nitrate (KNO₃) and 0.5%silicic acid (H₂SiO₃), at a temperature of 450° C.

The glasses constitutive of the lens molds according to the inventionare therefore advantageously strengthened by such a chemical temperingmethod known per se. Within the context of the present invention, theinventors have most particularly adapted such a chemical temperingmethod to the specific glasses constitutive of the lens molds accordingto the invention. The inventors recommend therefore, according to aparticularly advantageous variant, to carry out the chemical temperingwith these specific glasses under the conditions below:

in a potassium or (and) sodium nitrate bath; and in a particularlypreferred manner, in a potassium nitrate bath;

at a temperature between 425 and 475° C.; and in a particularlypreferred manner, at a temperature between 440 and 450° C.;

for 10 to 20 hours; and in a particularly preferred manner for 12 to 20hours (typically for 16 hours).

The thus thermally or chemically (advantageously chemically) treatedglasses can have a shallow compression layer of depth greater than orequal to 70 μm. Such glasses, with such a compression layer areparticularly preferred. They have a mechanical resistance which iscomparable, even better than that of prior art glasses known under thecode name TRC 33 (mechanical resistance evaluated by MOR on unabrasedsamples).

It is now suggested to revert back in greater detail to the weightcomposition of the glasses constitutive of the lens molds according tothe invention; the original composition which especially allows saidglasses to respond as favorably to the thermal and chemical temperingmethods.

Silica, SiO₂, is the oxide which forms the network of the glass and itintervenes between 56 and 66% by weight in the composition of theseglasses. If it intervenes in too low an amount (<56%), the glass becomessusceptible to deterioration; if it intervenes in too great an amount(>66%), the glass becomes difficult to melt. The silica content of theseglasses is advantageously between 61.5 and 63% by weight.

It is specified here in a general manner that the preferred ranges,indicated for the content of each one of the constituents are on the onehand preferred, in themselves, i.e. independent of the preferred rangesindicated for the other constituents and on the other hand, particularlypreferred, taken in combination with said preferred ranges for the otherconstituents.

Al₂O₃ enables:

improving the chemical resistance of these glasses;

increasing the ionic exchange kinetics which takes place between analkali of the shallow layer of said glasses and an alkali of a higherionic level, and thus enables increasing the depth of the compressionlayer generated during the thermal or chemical tempering. Al₂O₃intervenes between 2.5% and 10% by weight, above which value the glassbecomes very viscous and difficult to melt. Preferably, the glassesconstitutive of the lens molds according to the present invention havean Al₂O₃ content between 2.5 and 4%.

B₂O₃ enables improving the melting of these glasses. Said glassescontain between 0.5 and 7% thereof by weight. An excessive B₂O₃ contentis detrimental to the durability of the glass as well as for themaintenance of the strain point above 495° C. Preferably, the B₂O₃content is between 0.5 and 3% by weight.

The alkali oxides act as flux during the melting of the glass, and theyare therefore indispensable components for preparing said glass. Na₂O isan essential component for the ionic exchange which takes place duringthe chemical tempering advantageously carried out. Its content isbetween 8 and 15% by weight in the glasses. If its content is too high,the chemical deteriorability appears and the strain point decreases.

The presence of K₂O in the composition of the glasses enables improvingthe ionic exchange rate and controlling the thickness of the compressionlayer by altering the proportion between the two oxides Na₂O and K₂O.Moreover, its introduction in the composition of the glasses instead ofNa₂O enables decreasing the corrosion of the surface after chemicaltempering. K₂O is present between 3 and 12% by weight and preferablybetween 5 and 10%.

Li₂O may also be present in the composition, and this so as to increasethe level of compression in the layer by exchange between the lithiumand the potassium of the chemical tempering bath. The Li₂O content isbetween 0 and 3% by weight and preferably between 0 and 0.5% by weight.

The total alkali content (Li₂O+Na₂O+K₂O) in the composition of saidglasses is maintained between 12 and 20% by weight especially so as tocontrol the level of compression, the depth of the compression layer andthe durability before and after chemical tempering.

ZnO enables improving the melting of the glasses as well as theirviscosity, this without interfering with the eventual chemical temperingcarried out. ZnO is present at the rate of 2 to 12% by weight,advantageously at the rate of 7 to 12% by weight.

TiO₂ may intervene in the composition of the glasses especially so as toimprove their chemical durability. However, its presence at a contentgreater than 0.5% by weight induces an absorption in the UV between 310nm and 400 nm, which is detrimental to the recommended use of saidglasses (detrimental to the UV exposure treatment of the organic lenseswhich takes place across the glass molds of the invention). TiO₂ istherefore always present in a content lower than or equal to 0.5%.Advantageously, it is excluded from the composition of the glassesconstitutive of the lens molds according to the invention.

ZrO₂ is an oxide which enables improving the chemical durability of theglasses and especially their alkali durability and their hydrolyticresistance. A minimum of 1% by weight is necessary for taking advantageof this effect in the glasses. If the ZrO) content is too great, themelting of the glass becomes very difficult. The ZrO₂ content istherefore lower than or equal to 9%, and is preferably between 1 and 7%by weight.

The alkaline-earth elements, CaO, MgO, SrO, BaO have act as flux in ananalogous way to the alkalis; this is the reason why they mayadvantageously be present in the glasses, so as to improve the meltingand the forming of said glasses. But, insofar as CaO has a tendency todeteriorate the chemical strengthenability, its content remains between0 and 1% by weight. The MgO content is between 0 and 3% by weight andthose of BaO and SrO between 0 and 2% by weight.

The total CaO+MgO+BaO+SrO is between 0 and 5% by weight.

Cl is optionally present to improve the melting of the glass and tocontribute to its finishing in contents between 0 and 0.5% by weight aswell as other finishing agents, As₂O₃ and/or Sb₂O₃ which are themselvesoptionally present at a total content between 0 and 1% by weight. Theintervention of other finishing agents (such as Br, F and/or SO₃, forexample) is in no way excluded from the context of the presentinvention.

To all useful ends, it is hereby specified that, for what relates to theoptional components (Li₂O, MgO, TiO₂, CaO, BaO, SrO, Cl, As₂O₃, Sb₂O₃),the minimal intervening amount from which they exert a significanteffect is generally in the order of 0.5%. Thus, the glasses constitutiveof the lens molds according to the invention cannot contain saidconstituents or, if they contain them, it is generally in a minimalamount of 0.5% (% by weight).

The said glasses essentially consist of the constituents indicatedabove. It would not however be totally excluded that they contain otherconstituents within them. Such constituents can in any case intervene inlow amounts and have not a significant influence on the propertiessought after.

The said glasses, such as described above, are, as already indicated,transparent white glasses. Their properties are specified below. Theyare characterized:

by a good transmission in the UVs. Thus, their transmission at 315 nm isbetter than that of the BL glass, the reference in this field;

by a lower sensitivity to solarization under UV. The inventors havetested the said glasses in keeping them exposed for 200 hours under aXenon lamp or a Mercury/Xenon lamp. In both cases, the inventors havebeen able to verify, by measuring the transmission, before and afterexposure, that said glasses were not darkened. This is particularlyimportant from the point of view of the application of said glassessought after. In fact, the UV crosslinking of polymerizable compositionscast in glass molds (through the walls of said molds) is the technologywhich is carried out more and more for the production of organic lenses(by polymerization);

by the physical properties below:

strain point; >495° C.,

softening point: <810° C.;

CTE: 80-95×10⁻⁷/° C.;

density: <2.8;

Young's Modulus: >70,000 Mpa;

Liquidus viscosity: >10³ Pa·s (10,000 poises).

 (Such Liquidus viscosity values are particularly interesting insofar asthey make it possible to make the glasses by the standard industrialtechnologies);

by a very good chemical durability, which is greater than that of the loprior art glasses known under the code names QE-8092 and TRC 33 (Corningglasses used for organic lens molds, vide supra) and comparable to thatof the standard white “crown” BL glass (also vide supra) which itselfalso can be thermally strengthened. Said chemical durability has beenmeasured. The results below have been obtained for the said glasses:

acid durability (evaluated according to the standard DIN 12116 (videinfra)): on glasses tempered chemically in a KNO₃ bath at 440° C. for 16hours, the weight loss is less than 3 mg/dm²; it is 1 mg/dm² for certainpreferred glasses;

alkali durability (evaluated according to the standard NF B35602 (videinfra)): on glasses tempered chemically (under the same conditions), theweight loss is less than 200 mg/dm²;

hydrolytic resistance (evaluated according to the standard NF B25601(vide infra)): the weight loss is less than 150 mg/dm²;

by their thermal or chemical strengthenability: this has already beendeveloped earlier on in the present text.

The manufacture of these glasses constitutive of the lens moldsaccording to the invention does not present any particular difficulty;it does not require any unusual conditions or measures. The manufactureis within the reach of the person skilled in the art.

The classical starting materials, such as oxides, carbonates andnitrates, can be used for preparing fillers to be melted. The usualprecautions, as for the purity of said intervening starting materials,in order to obtain optical glasses suffice (obviously if it is desiredto obtain glasses of optical quality).

To all useful ends, with reference to the manufacture of these glasses,the following can be given in an illustrative manner. The valuesindicated for the parameters of the method correspond to the operatingmethod carried out for preparing the glasses in the Examples below.These values are in no way limiting. The constituents of the glasses canbe brought about by the starting materials specified below:

Oxides Starting materials SiO₂ Quartz, Sand Al₂O₃ Hydrated alumina B₂O₃B(OH)₃ ZrO₂ ZrO₂ TiO₂ TiO₂ CaO CaCO₃, Ca(NO₃)₂ SrO SrCO₃ BaO BaCO₃,Ba(NO₃)₂ MgO MgCO₃ ZnO ZnO Li₂O Li₂CO₃ Na₂O Na₂CO₃, NaNO₃ K₂O K₂CO₃,KNO₃

The starting materials selected preferably contain a content oftransition metal, particularly Fe₂O₃ and TiO₂, less than 160 ppm inorder that the glass obtained has a very interesting transmission in theUV and the visible.

After weighing, the various starting materials are mixed according tocommon techniques. The mixture is then placed in the oven in a platinumcrucible at a temperature of about 1,400° C.; when it is perfectlymolten, the temperature of the bath is brought to about 1,500-1,550° C.for the homogenization of the finishing. The bath of the glass is thencooled to a temperature corresponding to the viscosity which is adequatefor forming the glass. This glass is then cast in the form of a bar.

The total duration of the operation is in the order of 2 to 7 hours.After forming, the glass is baked again at about 650° C. with a coolingrate of 60° C./hour.

According to another of its objects, the present invention relates tothe use of the glasses having the compositions specified above and whichhave advantageously been strengthened (by a chemical or thermaltempering) for the production of organic lens molds. Said organic lensmolds according to the invention are perfectly appropriate, as indicatedfurther on in the present text, for preparing said lenses bypolymerization of a polymerizable composition cast within them; thepolymerization (cross-linking) carried out under UV irradiation throughthe walls (in specific glass of the invention) of said molds.

Among the glasses having the compositions specified above-glassesparticularly suitable as constitutive elements of organic lensmolds—some are novel and, as already indicated, constitute a furtherobject of the invention. The said further object consists of theinorganic glasses having the following composition A, and advantageouslythe following composition B, expressed in percentages by weight:

A B SiO₂ 56-66 61.5-63   Al₂O₃ 2.5-4   2.5-4   B₂O₃ 0.5-7   0.5-3   Li₂O0-3   0-0.5 Na₂O  8-15  8-15 K₂O  3-12  5-10 with Li₂O + Na₂O + K₂O12-20 ZnO  2-12  7-12 MgO 0-3 0-3 TiO₂   0-0.5 ZrO₂ 1-9 1-7 CaO 0-1 0-1BaO 0-2 0-2 SrO 0-2 0-2 with MgO + CaO + SrO ÷ BaO 0-5 Cl   0-0.5  0-0.5 As₂O₃ + Sb₂O₃ 0-1  0-1.

All the precisions generally given above in reference to the (novel ornot novel) glasses constitutive of the lens molds according to theinvention obviously apply to the novel ones, which are claimed per se.It must consequently be understood that the following novel glasses arepreferred

those of composition A which are TiO₂-free;

those of composition A or B which have been strengthened by a chemicaltempering or heat tempering, advantageously a chemical tempering;

those of composition A or B which have been strengthened by a chemicaltempering carried out in a potassium or (and) sodium nitrate bath,advantageously potassium nitrate bath, at a temperature between 425° C.and 475° C., for 10 to 20 hours;

those of composition A or B which have a shallow compression layer ofdepth at least equal to 70 μm.

The invention is illustrated in an entirely non-limiting manner byExamples 4 to 14 below. Examples 4 and 14 are particularly preferred.The glasses corresponding thereto belong to the preferred field.Examples 1 to 3 are given by way of comparison. They relate to:

the prior art standard white “crown” BL glass (Example 1);

the prior art TRC 33 glass (Example 2);

a glass without zirconium (Example 3), respectively.

The glasses whose compositions are given in Table 2 below have beenprepared according to the operating method specified above (thisoperating method was carried out on a laboratory scale. It is entirelyobvious that the glasses can be manufactured industrially by usingconventional methods of melting and forming). Said compositions areexpressed in percentages by weight.

TABLE 2 Example Composition (% by weight) 1 2 3 4 5 6 7 8 9 10 11 12 1314 SiO₂ 69.9 64.9 62.97 62.72 61.82 61.74 63.84 62.84 65.18 60.05 58.3156.61 61.5 62.1 Al₂O₃ 0.07 2.7 2.8 2.79 2.77 3.75 9.3 9.24 2.7 2.74 2.712.68 2.64 3.71 B₂O₃ 0.55 3 0.955 0.95 0.95 0.96 6.63 6.59 3 0.94 0.920.91 2.94 0.95 Li₂O 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Na₂O 11.25 10.18 8.448.41 8.36 11.36 15.18 15.09 10.26 8.27 8.18 8.09 10.02 10.41 K₂O 5 8.459.39 9.35 9.3 5.12 3.8 3.77 8.54 9.2 9.09 9 8.34 7.35 ZnO 0 7 11.7611.71 11.64 11.84 0 0 7.03 11.51 11.38 11.26 6.87 9.71 MgO 1 0 2.95 2.942.92 2.97 0 0 0 2.89 2.86 2.82 0 2.94 TiO₂ 0 1.5 0.73 0 0 0 0 0 0 0 0 00 0 ZrO₂ 0 0 0 1.12 2.23 2.27 1.24 2.47 2.29 4.41 6.55 8.63 6.71 3.24CaO 9 1 0 0 0 0 0 0 0.99 0 0 0 0.97 0 Sb₂O₃ 0.6 0.6 0.6 0.6 0.6 0.6 0.60.6 0.6 0.6 0.6 0.6 0.6 0.6

The physical and chemical properties of these glasses were determinedaccording to the following protocols.

After chemical tempering, the level of compression attained iscontrolled by an optical method on a thin blade by measuring the depthof the layer put under compression, expressed in microns, as well as thelevel of compression on the surface, expressed in PSI. A mechanicalcontrol by measuring the rupture modulus (MOR) (flexion in 3 points) onthe polished sample of 32 mm diameter, depth 3 mm has also been effected(measurement expressed in MPa).

The measurements of the refractive index and the Abbe number are carriedout according to the usual methods (for nd, the yellow ray of He isused) on re-baked samples.

The density is measured with the aid of a Micromeritics heliumpicnometer.

The elasticity modulus (Young's modulus) and the Poisson coefficient ofthese glasses have also been measured.

The U.V./Visible transmission from 300 to 800 nm is determined on apolished sample of 2 mm depth with the aid of a Perkin-Elmer Lambda 9spectrophotometer.

The acid durability is evaluated by the test of the standard DIN 12116.It consists in determining the weight loss of a polished sample,immersed for 3 hours in a 20% hydrochloric acid solution at 100° C. Theweight loss is expressed in mg/dm².

The results are expressed in classes:

Class 1: loss of less than 0.7 mg/dm²

Class 2: loss of 0.7 to 1.5 mg/dm²

Class 3: loss of more than 1.5 mg/dm².

The alkali durability is evaluated by the test of the standard NFB35602. It consists in determining the weight loss of a polished sampleimmersed for 3 hours in a mixture in equal proportions of normalsolutions of NaOH and Na₂CO₃.

Class 1: loss of less than 75 mg/dm²

Class 2: loss of 75 to 150 mg/dm²

Class 3: loss of more than 150 mg/dm².

The hydrolytic resistance is evaluated by the test of the standard NFB35601. The glass is ground (grains of 300 to 420 microns) andmaintained in distilled water at 100° C. for one hour. The alkalis arethen determined and expressed in μg/g of glass.

Class 1: loss of less than 30 μg/g of glass

Class 2: loss of 30 to 60 μg/g of glass

Class 3: loss of 60 to 260 μg/g of glass

Class 4: loss of 260 to 600 μg/g of glass

Class 5: loss of more than 600 μg/g of glass.

The deteriorabilities of the glasses, notably acid and alkalidurabilities, were characterized on the glasses before and afterchemical tempering.

Said physical and chemical properties of said glasses of the inventionaccording to Examples 4 to 14 (as a matter of fact it must be understoodthat only novel glasses according to Examples 4-6, 9-14 are actuallyglasses of the invention; while glasses according to Examples 7 and 8are prior art glasses suitable as constitutive glasses for lens moldsaccording to the invention) as well as those of the glasses according toExamples 1 to 3 are given in the Table 3 below.

In considering the values indicated in said Table, the person skilled inthe art immediately appreciates the interest of the invention.

TABLE 3 Example Properties 1 2 3 4 5 6 7 8 9 10 11 12 13 14 CTE*10⁻⁷/°C. 96 95 94.7 94.2 93.1 88.8 86.8 86.7 90.1 87.9 85.6 89.9 85.2 StrainPoint 497 486 502 510 521 512 505 502 493 533 550 563 531 529 (° C.)Annealing 539 528 550 557 568 560 547 544 536 578 595 607 572 576 Point(° C.) Softening 714 703 747 757 769 758 712 707 719 782 798 807 771 776Point (° C.) Durability be- fore chem. tempering Acid DIN 2 1 1 1 1 1 23 2 3 3 2 2 12116 class weight loss 0.8 0.1 0.3 0.3 0.6 0.4 1.2 3 1.21.7 1.8 1.2 1 (mg/dm²) Alkali NF 2 2 2 1 1 1 2 1 1 1 1 1 1 B35602 classweight loss 83 165 93.1 49.1 41.5 31.8 96.2 70.6 64 42.3 35.3 25.4 29.4(mg/dm²) Hydrolytic NF 3 3 3 3 3 3 3 3 3 3 3 3 3 3 B35601 class weightloss 210 168 119.9 103.3 88.9 89.9 80.6 74.4 127 86 88 68 100 68.2(μg/g) Mechanical properties E(Gpa) 73.6 71.5 70.5 71 71.3 72.5 75.976.4 71.6 74 74 78 75.1 G(Gpa) 30 29 28.5 29 29 29.1 31.4 31.4 29.2 29.929.9 31.6 30.6 v 0.22 0.22 0.24 0.22 0.23 0.23 0.21 0.22 0.23 0.2390.236 0.235 0.228 d 2.54 2.56 2.61 2.62 2.63 2.64 2.48 2.5 2.575 2.672.71 2.75 2.65 Chemical tempering 440° C. 16 h 450° C. 450° C. 450° C.450° C. 440° C. 16 h 16 h 16 h 16 h 16 h MOR (MPa) 606 712 796 760 899674 627 646 820 896 988 861 934 delta 23 86 100 103 93 65 77 101 33 117113 121 117 Durability after chem. tempering 440° C. 16 h Acid class 1/23 2 2 weight loss 0.8-0.5 1.6 1 1.4 (mg/dm²) Alkali class 2 2 2 1 weightloss 165 110.2 83.4 56.6 (mg/md²⁾ Transmission (2 mm) Tvis 91.8 91.591.4 91.7 91.7 92.1 92.1 92 92.05 91.3 91.4 91.3 91.5 92 x 0.3104 0.31050.3106 0.3104 0.3104 0.3104 0.3105 0.3104 0.3104 0.3105 0.3105 0.31060.3104 0.3105 y 0.3167 0.317 0.3169 0.3166 0.3166 0.3167 0.3166 0.31660.3166 0.3167 0.3167 0.3168 0.3166 0.3167 T315 nm 62.7 0 50.1 84.7 84.783.9 86.5 86.7 87.02 82.1 82.4 80.8 85.6 83.9 T340 nm 86.8 30 84.6 90 9090.3 90.75 90.6 90.6 89.08 89.33 88.7 90.07 90

What is claimed is:
 1. Organic lens mold comprising at least oneinorganic glass comprising the following composition, expressed inpercentages by weight: SiO₂ 56- 66 Al₂O₃ 2.5-10  B₂O₃ 0.5-7   Li₂O 0- 3Na₂O   8- 15 K₂O   3- 12 with Li₂O + Na₂O + K₂O 12- 20 ZnO   2- 12 MgO0- 3 TiO₂   0- 0.5 ZrO₂ 1- 9 CaO 0- 1 BaO 0- 2 SrO 0- 2 with MgO + CaO +SrO + BaO 0- 5 Cl   0- 0.5 As₂O₃ + Sb₂O₃  0-
 1.


2. Mold according to claim 1, wherein said inorganic glass is TiO₂-free.3. Mold according to claim 2, wherein said inorganic glass has thefollowing composition, expressed in percentages by weight: SiO₂61.5-63   Al₂O₃ 2.5-4   B₂O₃ 0.5-3   Li₂O   0-0.5 Na₂O  8-15 K₂O  5-10ZnO  7-12 MgO 0-3 ZrO₂ 1-7 CaO 0-1 BaO 0-2 SrO 0-2 Cl   0-0.5 As₂O₃ +Sb₂O₃  0-1.


4. Mold according to claim 3, wherein said inorganic glass has acompression layer of depth at least equal to 70 μm.
 5. Mold according toclaim 1, wherein said inorganic glass has been strengthened by achemical tempering or heat tempering.
 6. Mold according to claim 5,wherein said inorganic glass has been strengthened by a chemicaltempering carried out in a potassium and/or sodium nitrate bath.
 7. Moldaccording to claim 6, wherein said inorganic glass has been strengthenedby a chemical tempering carried out in a potassium nitrate bath at atemperature between 425° C. and 475° C. for 10 to 20 hours.
 8. Moldaccording to claim 7, wherein said inorganic glass has a compressionlayer of depth at least equal to 70 μm.
 9. Mold according to claim 1,wherein said inorganic glass has a compression layer of depth at leastequal to 70 μm.
 10. Mold according to claim 1, wherein said inorganicglass has the following composition, expressed in percentages by weight:SiO₂ 56- 66 Al₂O₃ 2.5-4   B₂O₃ 0.5-7   Li₂O 0- 3 Na₂O   8- 15 K₂O   3-12 with Li₂O + Na₂O + K₂O 12- 20 ZnO   2- 12 MgO 0- 3 TiO₂   0- 0.5 ZrO₂1- 9 CaO 0- 1 BaO 0- 2 SrO 0- 2 with MgO + CaO + SrO + BaO 0- 5 Cl   0-0.5 As₂O₃ + Sb₂O₃  0-
 1.


11. Method for producing organic lens molds, said method comprising:providing an inorganic glass as defined in claim 1, and forming saidinorganic glass into a mold.